Fibrous copper microparticles and process for producing same

ABSTRACT

The present invention provides fibrous copper microparticles suppressed in the occurrence of irregularities on the surface thereof and the aggregates of the fibrous copper microparticles having an average crystallite diameter controlled so as to fall within a specific range. In the fibrous copper microparticles of the present invention, the number of fibrous copper microparticles each including one or more irregularities each having a dimensional difference of 0.02 μm or more, in a range of 1 μm in the lengthwise direction of a fibrous body, between the maximum diameter portion of the fibrous body and the minimum diameter portion of the fibrous body falling in a diameter dimension range of 0.01 to 0.5 μm, and each having a length of 1 μm or more is 10 or less per 100 of the fibrous copper microparticles.

TECHNICAL FIELD

The present invention relates to fibrous copper microparticlessuppressed in the occurrence of one or more irregularities on thesurface of each of the fibrous copper microparticles and a process forproducing the same, and the aggregates of fibrous copper microparticleshaving an average crystallite diameter controlled so as to fall within aspecific range and a process for producing the same.

BACKGROUND ART

Copper microparticles are excellent in electrical conductivity, are aninexpensive material as compared with silver or the like, and hence arewidely used as raw materials for electrically conductive coating agentsand the like. Such electrically conductive coating agents are widelyused in materials for forming circuits on printed wiring boards or thelike by using various printing methods, and various electrical contactmembers.

Various investigations have been made on metal microparticle includingcopper microparticles, and processes for producing the metalmicroparticles. For example, fibrous copper microparticles andaggregates thereof, and processes for producing these (Patent Literature1 and Non Patent Literature 1) have been proposed.

CITATION LIST Patent Literature

-   Patent Literature 1: International Publication No. WO 2011/071885

Non Patent Literature

-   Non Patent Literature 1: Nano Lett., 2012, 12, pp. 234-239 (ACS    Publications, Dec. 15, 2011)

SUMMARY OF INVENTION Technical Problem

The fibrous copper microparticles and the aggregate thereof in PatentLiterature 1 are excellent in electrical conductivity, and hence arepromising as raw materials for various electrically conductivematerials. However, a large number of irregularities occur on thesurface of each of the fibrous copper microparticles of PatentLiterature 1. When such fibrous copper microparticles are used as theraw materials for various electrically conductive materials, it isanticipated that various industrial troubles due to the irregularitieswill occur.

By controlling the average crystallite diameter of the aggregates of thefibrous copper microparticles, the number of the crystallites per unitlength of the fibrous copper microparticles constituting the aggregatescan be controlled, accordingly, the interfaces between the crystallitescan be reduced, and thus, the electrical conductivity of the aggregatesof the fibrous copper microparticles or the like can be improved.However, Patent Literature 1 has not made a study on the preparation ofthe aggregates of the fibrous copper microparticles controlled so as foraverage crystallite diameter to fall within a small diameter range or alarge diameter range.

Non Patent Literature 1 describes a preparation of fibrous coppermicroparticles and the aggregates thereof by using a continuous reactionvessel. The shapes of the fibrous copper microparticles obtained by acontinuous reaction are suggested to be somewhat different in fiberlength or fiber diameter as compared with the shapes of the fibrouscopper microparticles obtained by a non-continuous reaction.

However, in Non Patent Literature 1, no remarks have been made on theoccurrence of one or more irregularities on the surface of each of thefibrous copper microparticles. In Non Patent Literature 1, noinvestigation has been made on the provision of fibrous coppermicroparticles suppressed in the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles.

Moreover, in Non Patent Literature 1, no investigation has been made onthe control of the average crystallite diameter of the aggregates of thefibrous copper microparticles.

An object of the present invention is to provide fibrous coppermicroparticles suppressed in the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles and the aggregates of the fibrous copper microparticles,in order to solve such problems as described above in the related art.Another object of the present invention is to provide, on the basis ofthe suppression of the occurrence of the one or more irregularities onthe surface of each of fibrous copper microparticles, fibrous coppermicroparticles and the aggregates thereof having an average crystallitediameter controlled so as to fall within a specific range.

Yet another object of the present invention is to provide a process forproducing fibrous copper microparticles, the process being capable ofproducing by simple operations fibrous copper microparticles suppressedin the occurrence of one or more irregularities on the surface each ofthe fibrous copper microparticles, and a process for producing theaggregates of the fibrous copper microparticles; and to provide aprocess for producing the aggregates of the fibrous coppermicroparticles, the process being capable of producing by simpleoperations the aggregates of the fibrous copper microparticles havingthe average crystallite diameter controlled so as to fall within aspecific range, on the basis of the suppression of the occurrence of theone or more irregularities on the surface of each of the fibrous coppermicroparticles.

Solution to Problem

The present inventors made a diligent study in order to solve theforegoing problems, and consequently have perfected the presentinvention by discovering for the first time the fibrous coppermicroparticles (namely, the fibrous copper microparticles suppressed inthe occurrence of one or more irregularities on the surface of each ofthe fibrous copper microparticles) wherein the number of fibrous coppermicroparticles each including one or more irregularities having adimensional difference of 0.02 μm or more, in the range of 1 μm in thelengthwise direction of a fibrous body, between the maximum diameterportion of the fibrous body and the minimum diameter portion of thefibrous body falling in a diameter dimension range of 0.01 to 0.5 μm,and each having a length of 1 μm or more is controlled so as to be aspecific proportion, and by discovering for the first time theaggregates of the fibrous copper microparticles.

The present inventors have perfected the present invention bydiscovering for the first time the aggregates of the fibrous coppermicroparticles having the average crystallite diameter controlled so asto fall within a specific range, on the basis of the suppression of theoccurrence of one or more irregularities on the surface of each of thefibrous copper microparticles.

The present inventors have perfected the process for producing thefibrous copper microparticles of the present invention, by discoveringfor the first time that fibrous copper microparticles each suppressed inthe occurrence of one or more irregularities on the surface of each ofthe fibrous copper microparticles are capable of being produced bysimple operations, without needing complicated operations.

The present inventors have perfected the process for producing theaggregates of the fibrous copper microparticles of the presentinvention, by discovering for the first time that it is possible toproduce by simple operations, without needing complicated operations,the aggregates of the fibrous copper microparticles having the averagecrystallite diameter controlled so as to fall within a specific range onthe basis of the suppression of the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles.

Specifically, the gist of the present invention is as follows.

(1) Fibrous copper microparticles, wherein the number of fibrous coppermicroparticles each including one or more irregularities each having adimensional difference of 0.02 μm or more, in a range of 1 μm in thelengthwise direction of a fibrous body, between the maximum diameterportion of the fibrous body and the minimum diameter portion of thefibrous body falling in a diameter dimension range of 0.01 to 0.5 μm,and each having a length of 1 μm or more is 10 or less per 100 of thefibrous copper microparticles.

(2) The fibrous copper microparticles according to (1), wherein for eachof the fibrous copper microparticles, the fiber diameter is 0.01 to 0.5μm, and the aspect ratio is 10 or more.

(3) Aggregates of fibrous copper microparticles, formed by allowing thefibrous copper microparticles according to (1) or (2) to aggregate,wherein the average crystallite diameter is 0.045 to 0.1 μm and theaverage fiber diameter is 0.05 to 0.15 μm.

(4) Aggregates of fibrous copper microparticles, formed by allowing thefibrous copper microparticles according to (1) or (2) to aggregate,wherein the average fiber diameter is 0.05 to 0.15 μm, and the averagecrystallite diameter of the aggregates is 0.45 or more times the averagefiber diameter.

(5) Aggregates of fibrous copper microparticles formed by allowing thefibrous copper microparticles according to (1) or (2) to aggregate,wherein the average crystallite diameter is 0.015 to 0.03 μm, and theaverage fiber diameter is 0.03 to 0.1 μm.

(6) A process for producing fibrous copper microparticles, wherein theproduction process is a process for producing the fibrous coppermicroparticles according to (1) or (2); and the production processincludes the following step (I) and the following step (II) or (III), inthis order:

the step (I) of heating, to 50 to 100° C., an aqueous solutioncontaining copper ion, an alkaline compound, a nitrogen-containingcompound capable of forming a stable complex with copper ion and areducing compound;

the step (II) of maintaining for 20 minutes or more the temperature ofthe aqueous solution after passing through the step (I), andcontinuously precipitating the fibrous copper microparticles;

the step (III) of cooling the temperature of the aqueous solution afterpassing through the step (I) to decrease the temperature thereof by 20°C. over a period of time of 15 minutes or more from immediately afterthe start of cooling, and continuously precipitating the fibrous coppermicroparticles.

(7) The process for producing fibrous copper microparticles according to(6), wherein in the step (II) or (III), the reducing compound is furtheradded to the aqueous solution.

(8) A process for producing aggregates of fibrous copper microparticles,wherein the production process is a process for producing the aggregatesof the fibrous copper microparticles according to (3) or (4); and theproduction process includes the following step (I) and the followingstep (IIa), in this order, and continuously precipitates the fibrouscopper microparticles or the aggregates of the fibrous coppermicroparticles:

the step (I) of heating to 50 to 100° C. the aqueous solution containingcopper ion, an alkaline compound, a nitrogen-containing compound capableof forming a stable complex with copper ion and a reducing compound;

the step (IIa) of maintaining for 30 minutes or more the temperature ofthe aqueous solution after passing through the step (I).

(9) A process for producing aggregates of fibrous copper microparticles,wherein the production process is a process for producing the aggregatesof the fibrous copper microparticles according to (5); and theproduction process includes the following step (Ia) and the followingstep (IIIa), in this order, and continuously precipitates the fibrouscopper microparticles or the aggregates of the fibrous coppermicroparticles:

the step (Ia) of heating to 65 to 100° C. the aqueous solutioncontaining copper ion, an alkaline compound, a nitrogen-containingcompound capable of forming a stable complex with copper ion and areducing compound;

the step (IIIa) of decreasing the temperature of the aqueous solutionafter passing through the step (Ia) by 20° C. over a period of time of15 minutes or more from immediately after the start of cooling.

(10) The process for producing aggregates of fibrous coppermicroparticles according to (8), wherein in the step (IIa), the reducingcompound is further added to the aqueous solution.

(11) The process for producing aggregates of fibrous coppermicroparticles according to (9), wherein in the step (IIIa), thereducing compound is further added to the aqueous solution.

(12) The process for producing fibrous copper microparticles accordingto (6) or (7), wherein as the reducing compound, one or more selectedfrom ascorbic acid, erythorbic acid and glucose are used.

(13) The process for producing aggregates of fibrous coppermicroparticles according to any one of (8) to (11), wherein as thereducing compound, one or more selected from ascorbic acid, erythorbicacid and glucose are used.

Advantageous Effects of Invention

The fibrous copper microparticles of the present invention aresuppressed in the occurrence of one or more irregularities on thesurface of each of the fibrous copper microparticles, and hence, whenthe fibrous copper microparticles are used as raw materials forelectrically conductive materials such as electrically conductivecoating agents, electrical conductivity coats or electrical conductivityfilms, it is possible to prevent the occurrence of various industrialtroubles caused by the irregularities.

According to the present invention, it is possible to obtain theaggregates of the fibrous copper microparticles having the averagecrystallite diameter controlled so as to fall within a specific range onthe basis of the suppression of the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles. In particular, according to the present invention, it ispossible to obtain the aggregates of the fibrous copper microparticleshaving an average crystallite diameter controlled so as to fall within alarger range than conventional ranges relative to an average fiberdiameter (“the aggregates of the fibrous copper microparticles having anaverage crystallite diameter of 0.045 to 0.1 μm and an average fiberdiameter of 0.05 to 0.15 μm”; and “the aggregates of the fibrous coppermicroparticles having an average fiber diameter of 0.05 to 0.15 μm andan average crystallite diameter of 0.45 or more times the average fiberdiameter”).

The aggregates of the fibrous copper microparticles of the presentinvention having the average crystallite diameter and the average fiberdiameter respectively falling within the foregoing ranges are excellentin electrical conductivity.

According to the present invention, it is possible to obtain theaggregates of the fibrous copper microparticles having an averagecrystallite diameter controlled so as to fall within a smaller range(0.015 to 0.03 μm) than conventional ranges on the basis of thesuppression of the occurrence of one or more irregularities on thesurface of each of the fibrous copper microparticles.

The aggregates of the fibrous copper microparticles of the presentinvention having the average crystallite diameter controlled so as tofall within such a specific range as described above can expect adrastically wider range of applications as compared with the aggregatesof the fibrous copper microparticles having an average crystallitediameter far from being controlled in the average crystallite diameter.

According to the process for production of the present invention,fibrous copper microparticles suppressed in the occurrence of one ormore irregularities on the surface of each thereof and the aggregates ofthe fibrous copper microparticles can be easily produced by an extremelysimple process of continuously precipitating the fibrous coppermicroparticles in a reducing compound-containing aqueous solution.

According to the process for production of the present invention, afterthe suppression of the occurrence of one or more irregularities on thesurface each of the fibrous copper microparticles, the aggregates of thefibrous copper microparticles having the average crystallite diameterand the average fiber diameter controlled so as to respectively fallwithin the foregoing ranges can be easily produced without needingcomplicated operations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating the irregularities occurringon the surface of a fibrous copper microparticle.

FIG. 2 is a view obtained by observing with a digital microscope a statein which the fiber diameters, the lengths and the irregularities of thefibrous copper microparticles cannot be properly measured or evaluatedbecause the fibrous copper microparticles crowd.

FIG. 3 is a view obtained by observing with a digital microscope a statein which the fiber diameters, the lengths and the irregularities of thefibrous copper microparticles can be properly measured or evaluatedbecause the fibrous copper microparticles do not crowd.

FIG. 4 is a graph showing the reactivity of each of the various reducingcompounds with the dissolved oxygen in an alkaline aqueous solution.

FIG. 5 is a chart showing the base line of the peak for determining theaverage crystallite diameter obtained by subjecting to X-ray diffractionthe aggregates of the fibrous copper microparticles.

FIG. 6 is an electron microscopic observation view of the fibrous coppermicroparticles of the present invention in Example 13.

FIG. 7 is an electron microscopic observation view of the conventionalfibrous copper microparticles in Comparative Example 2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the fibrous copper microparticles of the present inventionare described in detail.

The fibrous copper microparticles of the present invention are thefibrous copper microparticles wherein the number of fibrous coppermicroparticles including irregularities having a dimensional differenceof 0.02 μm or more, in a range of 1 μm in the lengthwise direction of afibrous body, between the maximum diameter portion of the fibrous bodyand the minimum diameter portion of the fibrous body falling in adiameter dimension range of 0.01 to 0.5 μm, and each having a length of1 μm or more is 10 or less per 100 of the fibrous copper microparticles.In other words, the fibrous copper microparticles of the presentinvention are the fibrous copper microparticles wherein the occurrenceof one or more irregularities on the surface of each thereof issuppressed.

On the surface of each of the conventional fibrous coppermicroparticles, a large number of irregularities occur. The use of suchfibrous copper microparticles as raw materials for electricallyconductive materials causes various industrial problems to occur. Forexample, when the fibrous copper microparticles each having a largenumber of irregularities occurring on the surface thereof are used as araw material in an electrically conductive material such as anelectrically conductive coating agent, unfortunately there are obtainedonly electrically conductive materials poor in the adhesiveness to thesubstrate, the surface smoothness, the electrical conductivity, thetransparency and the like.

The present inventors have discovered for the first time that the use offibrous copper microparticles, for electrically conductive materials,each suppressed in the occurrence of one or more irregularities on thesurface thereof prevents the occurrence of such problems as describedabove, and provides industrially advantageous effects in electricallyconductive materials. In other words, the essential feature of thepresent invention is the suppression of the occurrence of one or moreirregularities on the surface of each of fibrous copper microparticles(namely, the control of the proportion of the fibrous coppermicroparticles each undergoing the occurrence of one or moreirregularities on the surface thereof so as to fall within a specificrange).

In the present invention, the number of the fibrous coppermicroparticles each having one or more irregularities per 100 fibrouscopper microparticles is required to be 10 or less, is preferably 8 orless and more preferably 5 or less, and it is most preferable that thefibrous copper microparticles each having one or more irregularities benot present at all. When the number of the fibrous copper microparticleseach having one or more irregularities per 100 fibrous coppermicroparticles exceeds 10, the use of such fibrous copper microparticlesas a raw material for an electrically conductive material such as anelectrically conductive coating agent causes such industrial problems asdescribed above.

The one or more irregularities defined in each of the fibrous coppermicroparticles of the present invention are described as follows withreference to FIG. 1.

As shown in FIG. 1, the irregularity 3 defined in the present inventionis defined as an irregularity in the fibrous copper microparticle 1,having a dimensional difference 6 of 0.02 μm or more, in a range 2 of 1μm in the lengthwise direction of a fibrous body, between the maximumdiameter portion 4 of the fibrous body and the minimum diameter portion5 of the fibrous body falling in a diameter dimension range of 0.01 to0.5 μm. Here, the dimensional difference 6 means the difference betweenthe radius of the maximum diameter portion 4 and the radius of theminimum diameter portion 5. The irregularity 3 occurs not only in thelateral portion of the fibrous copper microparticle 1 but also at theend of the fibrous copper microparticle 1.

In the present invention, in the case where the dimensional difference 6is less than 0.02 μm, or even in the case where the dimensionaldifference 6 is 0.02 μm or more, when the dimensional difference 6 ispresent outside the range of 1 μm in the lengthwise direction of thefibrous body, the irregularity involved is not regarded as theirregularity 3. When the fibrous copper microparticles 1 each having adimensional difference 6 not regarded as the irregularity 3 are used(for example, when used for various electrically conductive materials),various performances are not adversely affected and no industrialproblems are caused.

The minimum diameter in the minimum diameter portion 5 is the diameterin the cross section perpendicular to the fiber lengthwise direction ofthe fibrous copper microparticle 1, and the diameter at the minimumdiameter position in the range 2 of 1 μm in the lengthwise direction ofthe fibrous body. The diameter dimension of the minimum diameter portion5 is required to be 0.01 to 0.5 μm, and is preferably 0.01 to 0.1 μm.

The maximum diameter in the maximum diameter portion 4 is the diameterin the cross section perpendicular to the fiber lengthwise direction ofthe fibrous copper microparticle 1, and means the diameter having amaximum dimension in the range 2 of 1 μm in the lengthwise direction ofthe fibrous body. The lower limit of the diameter dimension in themaximum diameter portion 4 is 0.03 μm as determined from the relationwith the dimensional difference 6 between the maximum diameter portion 4and the minimum diameter portion 5.

In the present invention, the fiber diameter of the fibrous coppermicroparticle 1 is the diameter of the minimum diameter portion 5 in thetotal length of the fibrous copper microparticle 1. The fiber diameteris preferably 0.01 to 0.5 μm and more preferably 0.01 to 0.1 μm. Whenthe fiber diameter of the fibrous copper microparticles 1 exceeds 0.5μm, the use of the fibrous copper microparticles in various electricallyconductive materials causes problems in the transparency, thedispersibility or the like of the electrically conductive materials.When the fiber diameter is less than 0.01 μm, the electricalconductivity, the coatability or the like of an electrically conductivematerial using the fibrous copper microparticles as a raw materialundergoes the occurrence of problems.

The length (fiber length) of each of the fibrous copper microparticles 1is required to be 1 μm or more from the viewpoint of the assessment ofone or more irregularities 3. In particular, the length of the fibrouscopper microparticles 1 is preferably 5 μm or more and more preferably10 μm or more. When the length of the fibrous copper microparticles 1 isless than 1 μm, the electrical conductivity, the transparency or thelike of an electrically conductive material using the fibrous coppermicroparticles as a raw material undergoes the occurrence of problems.On the other hand, from the viewpoint of the handling of theelectrically conductive coating agent in the formation of anelectrically conductive coat or an electrically conductive film using asa raw material the fibrous copper microparticles 1, the length of thefibrous copper microparticles 1 preferably does not exceed 500 μm.

The aspect ratio (the length of the fibrous copper microparticles 1/thefiber diameter of the fibrous copper microparticles 1) of the fibrouscopper microparticles 1 is preferably 10 or more, more preferably 100 ormore and furthermore preferably 300 or more. When the aspect ratio ofthe fibrous copper microparticles 1 is less than 10, the transparency,the electrical conductivity or the like of various electricallyconductive materials using as raw materials the fibrous coppermicroparticles undergoes the occurrence of problems.

In the present invention, the number (proportion) of the fibrous coppermicroparticles 1 each having one or more irregularities 3 per 100 of thefibrous copper microparticles 1 is derived by the following method.

The aggregates of the fibrous copper microparticles 1 are observed byusing, for example, a transmission electron microscope (TEM), a scanningelectron microscope (SEM) or a digital microscope. Next, from theaggregates of the fibrous copper microparticles 1, 100 of the fibrouscopper microparticles 1 each having a length of 1 μm or more areselected, and the surfaces of 100 of these fibrous copper microparticles1 are observed. As described above, when in a fibrous coppermicroparticle 1, in a range of 1 μm in the lengthwise direction of thefibrous body, there is a minimum diameter portion 5 falling in adiameter dimension range of 0.01 to 0.5 μm, and the dimensionaldifference 6 (the difference between the radius of the maximum diameterportion 4 and the radius of the minimum diameter portion 5) between theminimum diameter portion 5 and the maximum diameter portion 4 is 0.02 μmor more, the fibrous copper microparticle 1 is determined to be afibrous copper microparticle 1 having a irregularity 3. By counting thenumber of the fibrous copper microparticles 1 each undergoing theoccurrence of one or more irregularities 3, it is possible to determinethe number of the fibrous copper microparticles 1 each undergoing theoccurrence of one or more irregularities 3 on the surface thereof inrelation to 100 of the fibrous copper microparticles 1.

The fiber diameter and the fiber length of the fibrous coppermicroparticles of the present invention can also be measured by usingsuch a TEM, SEM or digital microscope as described above. Specifically,the diameters (the diameters of the minimum diameter portions in thetotal lengths of the individual fibrous copper microparticles) of the100 fibrous copper microparticles each having a length of 1 μm or more,selected from the aggregates of the fibrous copper microparticles aremeasured, and the average value of the measured values can be taken asthe fiber diameter. The lengths of the 100 fibrous copper microparticleseach having a length of 1 μm or more, selected from the aggregates ofthe fibrous copper microparticles are measured, and the average value ofthe measured values can be taken as the fiber length. The aspect ratioof the fibrous copper microparticles of the present invention can bederived by dividing the fiber length determined as described above ofthe fibrous copper microparticles (the average value of the lengths ofthe 100 fibrous copper microparticles) by the fiber diameter determinedas described above of the fibrous copper microparticles (the averagevalue of the fiber diameters of the 100 fibrous copper microparticles).

When the fibrous copper microparticles of the present invention areobserved, in the case where the adjacent fibrous copper microparticlesoverlap each other and crowd each other in the aggregates of the fibrouscopper microparticles, it is impossible to accurately evaluate theshapes of the fibrous copper microparticles. In such a case, by using,for example, an ultrasonic disperser, the fibrous copper microparticlescrowding each other to an excessive degree are disentangled to besubjected to observation.

FIGS. 2 and 3 show the observation views of the degree of the mutualcrowding (possibility or impossibility of shape evaluation) of thefibrous copper microparticles as observed by using a digital microscope(“VHX-1000, VHX-D500/510,” manufactured by Keyence Corp.). FIG. 2 showsa state in which the fiber diameters, the fiber lengths and theirregularities defined in the present invention of the fibrous coppermicroparticles cannot be properly measured or evaluated because thefibrous copper microparticles crowd. FIG. 3 shows a state in which thefiber diameters, the fiber lengths and the irregularities defined in thepresent invention of the fibrous copper microparticles can be properlymeasured or evaluated because the adjacent fibrous copper microparticlesdo not crowd to an excessive extent. The magnification factor of each ofthe observation views of FIGS. 2 and 3 is approximately 10000.

A process for producing fibrous copper microparticles is described indetail.

The present inventors made a diligent study from various aspects inorder to obtain fibrous copper microparticles suppressed in theoccurrence of one or more irregularities on the surface of each thereof.Consequently, the present inventors have discovered for the first timethat by a production process including the following process (I) and thefollowing process (II) or (III) in this order, the number of the fibrouscopper microparticles each having one or more irregularities on thesurface thereof per 100 fibrous copper microparticles can be controlledto be 10 or less, namely “fibrous copper microparticles suppressed inthe occurrence of one or more irregularities on the surface of eachthereof can be easily produced”:

the step (I) of heating, to 50 to 100° C., an aqueous solutioncontaining copper ion, an alkaline compound, a nitrogen-containingcompound capable of forming a stable complex with copper ion and areducing compound

the step (II) of maintaining for 20 minutes or more the temperature ofthe aqueous solution after passing through the step (I), andcontinuously precipitating the fibrous copper microparticles

the step (III) of cooling the temperature of the aqueous solution afterpassing through the step (I) to decrease the temperature thereof by 20°C. over a period of time of 15 minutes or more from immediately afterthe start of cooling, and continuously precipitating the fibrous coppermicroparticles

The phrase, “continuously precipitating the fibrous coppermicroparticles” in the steps (II) and (III), means the consecutiveprecipitation of the fibrous copper microparticles by using a reactionvessel made of a glass tube or a stainless steel tube and by allowingthe aqueous solution to flow continuously.

Hereinafter, the individual components contained in the aqueous solutionof the step (I) are described.

The copper ion is a divalent cation capable of being produced bydissolving a water-soluble copper salt in water. Examples of thewater-soluble copper salt include: copper sulfate, copper nitrate,copper chloride and copper acetate. Among these, copper sulfate orcopper nitrate is preferable from the viewpoint of the easiness informing fibrous copper microparticles.

As the alkaline compound, without being particularly limited, forexample, sodium hydroxide or potassium hydroxide can be used.

The concentration of the alkaline compound in the aqueous solution ispreferably 10 to 50% by mass, more preferably 20 to 45% by mass andfurthermore preferably 20 to 40% by mass. When the concentration of thealkaline compound is less than 10% by mass, the fibrous coppermicroparticles are hardly formed. On the other hand, when theconcentration of the alkaline compound exceeds 50% by mass, the handlingof the aqueous solution is difficult.

The concentration of copper ion in the aqueous solution is specified bythe molar ratio between the hydroxide ion of the alkaline compound andcopper ion. Specifically, the concentration of copper ion in the aqueoussolution is preferably set so as for the ratio (hydroxide ion ofalkaline compound)/(copper ion) in terms of molar ratio to fall within arange preferably from 1500/1 to 6000/1 and more preferably from 1500/1to 5000/1. When the molar ratio is less than 1500/1, the shape of thecopper microparticles tends to be spherical without being fibrous. Onthe other hand, when the molar ratio exceeds 6000/1, the formationefficiency of the fibrous copper microparticles is degraded.

Examples of the nitrogen-containing compound include: ammonia,ethylenediamine and triethylenetetramine. Among these, ethylenediamineis preferable from the viewpoint of the easiness in forming the fibrouscopper microparticles.

The nitrogen-containing compound is preferably used in a proportion of 1mole or more in relation to 1 mole of copper ion from the viewpoint ofthe formation efficiency of the fibrous copper microparticles.

In the step (I), as the reducing compound beforehand contained in theaqueous solution heretofore known compounds can be used. Examples ofsuch compounds include: hydrogen gas; hydrogen compounds such ashydrogen iodide, hydrogen sulfide, lithium aluminum hydride and sodiumborohydride; lower oxides such as carbon monoxide, sulfur dioxide andsulfite or the salts of these; sulfur compounds such as sodium sulfide,sodium polysulfide and ammonium sulfide; metals such as alkali metals,magnesium, calcium and aluminum, or amalgams of these; and organiccompounds such as aldehydes, saccharides, formic acid, oxalic acid,hydrazine, ascorbic acid, erythorbic acid, glucose, amines and thiols.

Among these, as the reducing compound beforehand contained in theaqueous solution in the step (I), for example, ascorbic acid, erythorbicacid or glucose can be preferably used, and it is particularlypreferable to use ascorbic acid or erythorbic acid.

As the reducing compound beforehand contained in the aqueous solution inthe step (I), it is preferable to use “a reducing compound not reactingwith the dissolved oxygen in the alkaline aqueous solution” from theviewpoint of the capability of producing the fibrous coppermicroparticles sufficiently suppressed in the occurrence of one or moreirregularities on the surface of each thereof. When “a reducing compoundreacting with the dissolved oxygen in the alkaline aqueous solution” isused as the reducing compound, the proportion of the fibrous coppermicroparticles each having one or more irregularities on the surface ofeach thereof in the fibrous copper microparticles sometimes exceeds 10per 100 of the fibrous copper microparticles. In other words, it issometimes impossible to produce fibrous copper microparticlessufficiently suppressed in the occurrence of one or more irregularitieson the surface of each thereof.

“The reducing compound not reacting with the dissolved oxygen in thealkaline aqueous solution” in the present invention is defined by thefollowing index.

First, the dissolved oxygen concentration in the alkaline aqueoussolution is measured. The resulting dissolved oxygen concentration istaken as the dissolved oxygen concentration 1. Next, the reducingcompound is added to and dissolved in the alkaline aqueous solution.While the stirring is being continued successively even after thedissolution, the dissolved oxygen concentration at the elapsed time of10 minutes after the addition of the reducing compound is measured. Thedissolved oxygen concentration of this case is taken as the dissolvedoxygen concentration 2.

Then, the dissolved oxygen concentration retention rate is determined bythe following formula (1):

(Dissolved oxygen concentration retention rate)=(dissolved oxygenconcentration 2)/(dissolved oxygen concentration 1)  (1)

In the present invention, the reducing compound having the dissolvedoxygen concentration retention rate of 0.5 or more obtained by theformula (1) is defined as “the reducing compound not reacting with thedissolved oxygen in the alkaline aqueous solution.” And, the reducingcompound having the dissolved oxygen concentration retention rate ofless than 0.5 obtained by the formula (1) is defined as “the reducingcompound reacting with the dissolved oxygen in the alkaline aqueoussolution.”

FIG. 4 shows the relations between the dissolved oxygen concentration(mg/L) in the alkaline aqueous solution having a pH of 10.4 and the time(after the elapsed times of 0.5 minute, 5 minutes, 10 minutes, 15minutes, 30 minutes, 45 minutes and 60 minutes) for ascorbic acid,erythorbic acid, glucose and hydrazine as the reducing compounds.

As FIG. 4 shows, when ascorbic acid, erythorbic acid and glucose areused as the reducing compounds, the dissolved oxygen concentrationretention rates after the elapsed time of 10 minutes are 0.5 or more.Accordingly, in the present invention, ascorbic acid, erythorbic acidand glucose are each defined as “the reducing compound not reacting withthe dissolved oxygen in the alkaline aqueous solution.” As FIG. 4 shows,when ascorbic acid, erythorbic acid and glucose having a dissolvedoxygen concentration retention rate of 0.5 or more are used, highdissolved oxygen concentrations are maintained even after the elapsedtime of 30 minutes. The dissolved oxygen concentration retention rateobtained by the formula (1) is 0.90 for ascorbic acid, 0.96 forerythorbic acid and 0.97 for glucose.

On the other hand, as FIG. 4 shows, when hydrazine is used as thereducing compound, the dissolved oxygen concentration in the alkalineaqueous solution is rapidly and remarkably decreased, and the dissolvedoxygen concentration retention rate after the elapsed time of 10 minutesis less than 0.5. Accordingly, in the present invention, hydrazine isdefined as “a reducing compound reacting with the dissolved oxygen inthe alkaline aqueous solution.” For hydrazine, the dissolved oxygenconcentration retention rate obtained by the formula (1) is 0.03.

Such a reducing compound as described above is contained in a proportionsuch that the number of moles of the reducing compound is preferably 0.1to 10 times and more preferably 0.2 to 5 times the number of moles ofthe copper ion in the aqueous solution. When the proportion of thecontained reducing compound is less than 0.1 times the number of molesof the copper ion, the efficiency of the reduction reaction is degraded.On the other hand, even when the reducing compound is contained in aproportion exceeding 10 times the number of moles of the copper ion, thereduction reaction is saturated to be unfavorable from the viewpoint of,for example, the cost.

In the preparation of the aqueous solution containing such components asdescribed above, the individual components may be combinedsimultaneously and mixed by stirring. Alternatively, the aqueoussolution may also be prepared by adding the reducing compound after thecomponents other than the reducing compound are stirred and mixed.

Next, in the step (I), such an aqueous solution (blue aqueous solution)as described above is heated until the aqueous solution turns almostcolorless and transparent by using an appropriate heat source. In thiscase, the heating temperature is appropriately 50 to 100° C. The heatingcan use a method of continuous heating with a reaction vessel made of aglass tube or a stainless steel tube allowing the aqueous solution tocontinuously flow, or a method of heating the aqueous solution in anappropriate reaction vessel. In light of the simplicity in transition tothe next step (II), the former method of continuous heating ispreferable. This is because the continuous heating allows the aqueoussolution before reaction to be continuously fed, and the aqueoussolution after reaction to be continuously collected, and thus allowsthe continuous transition to the step (II).

The heating temperature of the aqueous solution in the step (I) set at50 to 100° C. allows the reaction efficiency and the controllability tobe excellent. In other words, when the heating temperature is lower than50° C., the reaction efficiency is degraded; when the heatingtemperature exceeds 100° C., it is difficult to control the shape or theprecipitation rate of the precipitated fibrous copper microparticles.The heating temperature is preferably 60 to 80° C.

In the step (I), when the aqueous solution is heated, the aqueoussolution may be stirred by an appropriate technique and underappropriate conditions.

In the related art, after the step (I), the fibrous coppermicroparticles are precipitated in a short time; however, when thefibrous copper microparticles are precipitated without passing through asufficient temperature maintenance time, it is impossible to producefibrous copper microparticles sufficiently suppressed in the occurrenceof one or more irregularities on the surface of each thereof.

Accordingly, in the present invention, the step (II) maintains thetemperature of the aqueous solution after passing through the step (I)for 20 minutes or more, and thus controls the occurrence of one or moreirregularities on the surface of each of the obtained fibrous coppermicroparticles so as to fall within the foregoing range. Here, “themaintenance of the temperature of the aqueous solution” means theoperation allowing the temperature of the aqueous solution not todecrease by 20° C. or more.

Moreover, in the present invention, the step (III) decreases thetemperature of the aqueous solution after passing through the step (I)by 20° C. over a time of 15 minutes or more, and thus, controls theoccurrence of one or more irregularities on the surface of each of theobtained fibrous copper microparticles so as to fall within theforegoing range.

In the step (II), while the aqueous solution is being continuouslyheated by using a reaction vessel allowing the aqueous solution to flowcontinuously, the temperature of the aqueous solution is maintained. Inthe step (II), for the purpose of maintaining the temperature of theaqueous solution or controlling the heating time of the aqueoussolution, for example, the size and the flow rate of the reaction vesseland/or the flow path may be appropriately selected.

Similarly, also in the step (III), the temperature of the aqueoussolution can be continuously decreased by using a reaction vesselallowing the aqueous solution to continuously flow.

In the steps (II) and (III), a further addition of the reducing compoundto the aqueous solution and the maintenance of the temperature of theaqueous solution allows the fiber diameters of the individualprecipitated fibrous copper microparticles to be made uniform, and alsoallows the fibrous copper microparticles to be obtained in a higherformation yield.

As the reducing compound to be added in the steps (II) and (III), aheretofore known reducing compound such as the reducing compoundcontained in the aqueous solution in the step (I) can be used, and “thereducing compound not reacting with the dissolved oxygen in the alkalineaqueous solution” as described above is preferable. The reducingcompound to be further added to the aqueous solution in the steps (II)and (III) may be of the same type as or of the type different from thetype of the reducing compound beforehand contained in the aqueoussolution in the step (I).

The addition amount of the reducing compound to be further added in thesteps (II) and (III) is preferably in terms of the number of moles 0.5to 100 times and more preferably 1 to 10 times the number of moles ofthe copper ion in the aqueous solution. When the addition number ofmoles of the reducing compound exceeds 100 times the number of moles ofthe copper ion, the formation effect of the fibrous coppermicroparticles is saturated to be unfavorable from the viewpoint of, forexample, the cost.

After passing through the steps (II) and (III), the precipitated fibrouscopper microparticles can be collected by performing solid-liquidseparation based on a method such as filtration, centrifugal separationor pressure floatation. Moreover, if necessary, for example, thecollected fibrous copper microparticles may also be washed or dried.When the fibrous copper microparticles are taken out, the surface of thefibrous copper microparticles tends to be oxidized, and hence thecollecting operation is preferably performed in an inert gas atmosphere(for example, nitrogen gas atmosphere).

When the taken-out fibrous copper microparticles are stored, the fibrouscopper microparticles are preferably stored in an atmosphere of an inertgas such as nitrogen gas, or preferably stored as redispersed, forexample, in a solution in which a trace amount of a reducing compound isdissolved or in a solution in which an organic substance having afunction to prevent the oxidation of copper is dissolved in a traceamount.

The aggregates of the fibrous copper microparticles of the presentinvention are described in detail.

The aggregates of the fibrous copper microparticles of the presentinvention are prepared by allowing the fibrous copper microparticlessuppressed in the occurrence of one or more irregularities on thesurface of each thereof to aggregate to each other as described above,and further by controlling the average crystallite diameter and theaverage fiber diameter so as to fall respectively within predeterminedranges as described below.

In the aggregates of the fibrous copper microparticles of the presentinvention, after the suppression of the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles as described above, the average crystallite diameter canbe controlled to be larger than the average crystallite diameter of theaggregates of the conventional fibrous copper microparticles.Specifically, the average crystallite diameter is 0.045 to 0.1 μm, andthe average fiber diameter is 0.05 to 0.15 μm. In particular, theaverage crystallite diameter is preferably 0.045 to 0.07 μm. The averagefiber diameter is preferably 0.05 to 0.08 μm.

“The crystallite” means the largest aggregate regarded as a singlecrystal present in the aggregates of the fibrous copper microparticles.

“The average fiber diameter of the aggregates of the fibrous coppermicroparticles” means the average value of the fiber diameters of theindividual fibrous copper microparticles constituting the aggregates.“The fiber diameter of the fibrous copper microparticle” means thediameter in the cross section perpendicular to the fiber lengthwisedirection in each of the fibrous copper microparticles. The method fordetermining the average fiber diameter of the aggregates of the fibrouscopper microparticles is described later.

In the aggregates of the fibrous copper microparticles having an averagecrystallite diameter of 0.045 to 0.1 μm and an average fiber diameter of0.05 to 0.15 μm, the number of crystallites per unit length of thefibrous copper microparticles constituting the aggregates can bereduced. The interfaces between the crystallites can be thereby reduced.The interfaces are the factors disturbing the electrical conductivity,and hence the reduction of the interfaces improves the electricalconductivity of the aggregates of the fibrous copper microparticles. Theaggregates of the fibrous copper microparticles having a large averagecrystallite diameter (0.045 to 0.1 μm) are excellent in stability andhardly undergo the effect of oxidation or the like.

Another aspect of the aggregates of the fibrous copper microparticles isthe aggregates being suppressed in the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles as described above, having an average fiber diameter of0.05 to 0.15 μm and having an average crystallite diameter of 0.45 ormore times the average fiber diameter. In particular, the aggregates ofthe fibrous copper microparticles having an average fiber diameter of0.05 to 0.08 μm are preferable. The average crystallite diameter ispreferably 0.6 or more times and more preferably 0.7 or more times theaverage fiber diameter.

Similarly, in the aggregates of the fibrous copper microparticles havingan average fiber diameter of 0.05 to 0.15 μm and an average crystallitediameter of 0.45 or more times the average fiber diameter, the number ofcrystallites per unit length of the fibrous copper microparticlesconstituting the aggregates can be reduced, and the electricalconductivity of the aggregates is improved.

On the other hand, in the aggregates of the fibrous coppermicroparticles, after the suppression of the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles as described above, the average crystallite diameter canbe controlled to be smaller than the average crystallite diameter of theaggregates of the conventional fibrous copper microparticles.Specifically, the average crystallite diameter is 0.015 to 0.03 μm, andthe average fiber diameter is 0.03 to 0.1 μm. In particular, the averagecrystallite diameter is preferably 0.015 to 0.025 μm. The average fiberdiameter is preferably 0.05 to 0.07 μm.

In the aggregates of the fibrous copper microparticles having an averagecrystallite diameter of 0.015 to 0.03 μm, namely, the aggregates of thefibrous copper microparticles having a small average crystallitediameter, the average fiber diameter of the aggregates tends to besmall. Examples of such aggregates include the aggregates of the fibrouscopper microparticles having an average fiber diameter of 0.03 to 0.1μm, and the aggregates of the fibrous copper microparticles having anaverage fiber diameter of 0.05 to 0.07 μm. The aggregates of the fibrouscopper microparticles having an average crystallite diameter of as smallas 0.015 to 0.03 μm are excellent in the reactivity with othersubstances, and can have drastically wide ranges of applications.

For example, when the average fiber diameter of the aggregates of thefibrous copper microparticles falls within a small range (for example, arange of 0.1 μm or less), the use of the aggregates of the fibrouscopper microparticles as a raw material for an electrically conductivematerial results in, for example, remarkably satisfactory transparencyof the electrically conductive material or remarkably satisfactoryadhesiveness to the substrate of the electrically conductive material.Accordingly, in the applications more requiring, for example, thetransparency and the adhesiveness to the substrate of electricallyconductive materials, aggregates of fibrous copper microparticles havingsmall average fiber diameters are preferably used.

As described above, in the individual fibrous copper microparticlesconstituting the aggregates of the fibrous copper microparticles, thereare no fine copper granules formed in a state of attaching to the endsor the lateral portions of such fibrous copper microparticles so as tobe integrated with the fibrous copper microparticles, or in a state ofbeing brought into contact with but not integrated with the fibrouscopper microparticles. When fine copper granules are formed, there arecrystal interfaces between the main portions of the fibers constitutingthe aggregates and the copper granules, and hence the electricalconductivity of the aggregates of the fibrous copper microparticles isdegraded.

Next, a method for deriving the average crystallite diameter of theaggregates of the fibrous copper microparticles is described below. Inthe present invention, the full width at half maximum (the width of thediffraction intensity curve at the intensity of half the peak intensity)of the peak (the peak for identification of copper) corresponding to the(111) plane of copper is determined by X-ray diffraction method, and byusing the full width at half maximum, the average crystallite diameterof the aggregates of the fibrous copper microparticles can bedetermined. Specifically, the aggregates of the fibrous coppermicroparticles are subjected to X-ray diffraction using a wide angleX-ray diffractometer “RINT-TTR IV” (manufactured by Rigaku Corp.), thusthe full width at half maximum β of the output peak corresponding to the(111) plane of copper is determined, and the average crystallitediameter is determined by substituting the full width at half maximum βinto the following formula (2):

(Average crystallite diameter) (μm)=(K×λ)/(β×cos θ)  (2)

In the formula (2), K is the Scherrer constant and has a value of 0.9, λrepresents the wavelength of the X-ray used, and θ represents thediffraction angle (2θ/θ) (rad).

The average length of the aggregates of the fibrous coppermicroparticles is preferably 1 μm or more and more preferably 5 μm ormore. When the average length is less than 1 μm, for example, theelectrical conductivity or the transparency of the electricallyconductive materials using the aggregates of the fibrous coppermicroparticles undergoes the occurrence of problems. On the other hand,from the viewpoint of the handling of the coating agent or the like inthe formation of an electrically conductive coat or an electricallyconductive film using the aggregates of the fibrous coppermicroparticles, the average length of the aggregates of the fibrouscopper microparticles is preferably 500 μm or less. “The average lengthof the aggregates of the fibrous copper microparticles” in the presentinvention is the average value of the lengths of the individual fibrouscopper microparticles constituting the aggregates. “The lengths of theindividual fibrous copper microparticles” means the lengths in the fiberlengthwise direction. The method for determining the average length ofthe aggregates of the fibrous copper microparticles is described later.

The average aspect ratio (average length/average fiber diameter) of theaggregates of the fibrous copper microparticles preferably having suchan average fiber diameter and an average length is preferably 10 ormore, more preferably 100 or more and furthermore preferably 300 ormore.

A method for determining the average fiber diameter and the averagelength of the aggregates of the fibrous copper microparticles isdescribed below.

By using, for example, a TEM, SEM or digital microscope, the aggregatesof the fibrous copper microparticles are observed. From the aggregates,100 fibrous copper microparticles are selected. The fiber diameter andthe length of each of these 100 fibrous copper microparticles aremeasured, and the average value of the fiber diameters and the averagevalue of the lengths for these 100 fibrous copper microparticles aretaken respectively as the average fiber diameter and the average lengthof the aggregates of the fibrous copper microparticles. The averageaspect ratio of the aggregates of the fibrous copper microparticles isderived by dividing the average length determined as described above bythe average fiber diameter determined as described above.

When the aggregates of the fibrous copper microparticles are observed,in the case where the fibrous copper microparticles constituting theaggregates overlap each other and crowd each other, it is impossible toaccurately evaluate the average fiber diameter and the average length ofthe aggregates of the fibrous copper microparticles. In such a case, byusing, for example, an ultrasonic disperser, the aggregates of thefibrous copper microparticles are disentangled to such an extent thatthe adjacent fibrous copper microparticles do not crowd each other, andthen the fibrous copper microparticles are observed.

A process for producing the aggregates of the fibrous coppermicroparticles is described in detail. Description is omitted for thesame constitution as in the process for producing the fibrous coppermicroparticles.

The present inventors made a diligent study from various aspects inorder to obtain the aggregates of the fibrous copper microparticlescapable of controlling the average crystallite diameter so as to fallwithin a predetermined range wherein the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles is suppressed as described above. Consequently, thepresent inventors have for the first time discovered that by aproduction process including the following step (I) and the followingstep (IIa) in this order, the aggregates of the fibrous coppermicroparticles having a large average crystallite diameter (0.045 to 0.1μm) can be easily produced wherein the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles is suppressed as described above:

the step (I) of heating to 50 to 100° C. the aqueous solution containingcopper ion, an alkaline compound, a nitrogen-containing compound capableof forming a stable complex with copper ion and a reducing compound

the step (IIa) of maintaining for 30 minutes or more the temperature ofthe aqueous solution after passing through the step (I)

The step (I) is the same as the step (I) of the process for producingthe fibrous copper microparticles.

The step (IIa) maintains for “30 minutes or more” the temperature of theaqueous solution after passing through the step (I). Otherwise, the step(IIa) is the same as the step (II) of the process for producing thefibrous copper microparticles.

When in the step (IIa), the time for maintaining the temperature of theaqueous solution in less than 30 minutes, the average crystallitediameter of the aggregates of the precipitated fibrous coppermicroparticles cannot be made sufficiently large. Specifically, theaverage crystallite diameter cannot be controlled so as to fall within arange from 0.045 to 0.1 μm.

On the contrary, when in the step (IIa), the time for maintaining thetemperature of the aqueous solution exceeds 480 minutes, the effect ofthe suppression of the occurrence of one or more irregularities on thesurface of each of the fibrous copper microparticles is saturated.Moreover, in this case, the average crystallite diameter in theaggregates of the fibrous copper microparticles cannot be made large soas to exceed 0.1 μm to be unfavorable from the viewpoint of, forexample, the cost.

On the other hand, the present inventors have for the first timediscovered that by a production process including the following step(Ia) and the following step (IIIa) in this order, the aggregates of thefibrous copper microparticles having a small average crystallitediameter (0.015 to 0.03 μm) can be easily produced wherein theoccurrence of one or more irregularities on the surface of each of thefibrous copper microparticles is suppressed as described above:

the step (Ia) of heating to 65 to 100° C. the aqueous solutioncontaining copper ion, an alkaline compound, a nitrogen-containingcompound capable of forming a stable complex with copper ion and areducing compound

the step (IIIa) of decreasing the temperature of the aqueous solutionafter passing through the step (Ia) by 20° C. over a period of time of15 minutes or more from immediately after the start of cooling.

In the step (Ia), the temperature for heating the aqueous solutioncontaining copper ion, an alkaline compound, a nitrogen-containingcompound capable of forming a stable complex with copper ion and areducing compound is set at “65 to 100° C.” Otherwise, the step (Ia) isthe same as the step (I) of the process for producing the fibrous coppermicroparticles.

When in the step (Ia), the temperature for heating the aqueous solutionis lower than 65° C., the average crystallite diameter of the aggregatesof the precipitated fibrous copper microparticles cannot be madesufficiently small. Specifically, the average crystallite diametercannot be controlled so as to fall within a range from 0.015 to 0.03 μm.

The step (IIIa) cools the aqueous solution after the step (Ia).Otherwise, the step (IIIa) is the same as the step (III) of the processfor producing the fibrous copper microparticles.

In the step (IIIa), the temperature of the aqueous solution passingthrough the step (Ia) is cooled not rapidly but gradually (the coolingby 20° C., over a time of 15 minutes or more), and thus it is possibleto obtain the aggregates of the fibrous copper microparticlessufficiently suppressed in the occurrence of one or more irregularitieson the surface of each thereof and having an average crystallitediameter controlled so as to fall within a range from 0.015 to 0.03 μm(preferably so as to fall within a range from 0.015 to 0.025 μm).

When in the step (IIIa), the cooling time is less than 15 minutes,namely, the aqueous solution is cooled not gradually but rapidly, theaggregates of the fibrous copper microparticles are not precipitated.

The fibrous copper microparticles of the present invention and theaggregates thereof are mixed with and dispersed in a binder component, asolvent and the like, and thus, an electrically conductive coating agentcan be prepared.

The binder component contained in the electrically conductive coatingagent is not particularly limited, and examples of the usable bindercomponent include: acrylic resins (such as acrylic silicone-modifiedresin, fluorine-modified acrylic resin, urethane-modified acrylic resinand epoxy-modified acrylic resin); polyester-based resin,polyurethane-based resin, olefin-based resin, amide resin, imide resin,epoxy resin, silicone resin and vinyl acetate-based resin; naturalpolymers such as starch, gelatin and agar; cellulose derivatives(semisynthetic polymers) such as carboxymethyl cellulose, hydroxy ethylcellulose, methyl cellulose, hydroxy ethyl methyl cellulose, hydroxypropyl methyl cellulose; and water-soluble polymers (synthetic polymers)such as polyvinyl alcohol, polyacrylic acid-based polymer,polyacrylamide, polyethylene oxide and polyvinylpyrrolidone.

The solvent contained in the electrically conductive coating agent isnot particularly limited, and examples of the contained solvent include:water, and organic solvent such as alcohols, glycols, cellosolves,ketones, esters, ethers, amides and hydrocarbons. These can be used eachalone or in combinations of two or more thereof. Among these, it ispreferable to use a solvent mainly composed of water or alcohols.

The mixing ratio between the fibrous copper microparticles or theaggregates thereof and the binder component in the electricallyconductive coating agent is, in terms of the volume ratio (A/B) betweenthe volume (A) of the fibrous copper microparticles or the aggregatesthereof and the volume (B) of the binder component, preferably 1/100 to5/1 and more preferably 1/20 to 1/1. When the amount of the fibrouscopper microparticles or the aggregates thereof is small to such anextent that the volume ratio of the fibrous copper microparticles or theaggregates thereof to the binder component is (less than 1)/100, theelectrical conductivity of, for example, the obtained electricallyconductive coating agent or the electrically conductive coat obtainedfrom the coating agent is degraded. On the other hand, when the amountof the binder component is small to such an extent that the volume ratiois (more than 5)/1, for example, the surface smoothness or thetransparency of the obtained electrically conductive coat suffers fromthe occurrence of problems, or when the obtained electrically conductivecoating agent is applied to a substrate, the adhesiveness to thesubstrate is degraded.

The solid content (the total content of the fibrous coppermicroparticles or the aggregates thereof of the present invention, thebinder component, and if necessary, the solid content of the otheradditive(s)) concentration in the electrically conductive coating agentis preferably 1 to 99% by mass and more preferably 1 to 50% by mass,from the viewpoint of being excellent in the balance, for example,between the electrical conductivity and the handleability.

The viscosity of the electrically conductive coating agent at 20° C. ispreferably 0.5 to 100 mPa·s and more preferably 1 to 50 mPa·s from theviewpoint of being excellent, for example, in the handleability and theeasiness in application to substrates.

In the electrically conductive coating agent, a cross-linking agent forcross-linking the binder component, such as an aldehyde-based,epoxy-based, melamine-based or isocyanate-based cross-linking agent mayalso be used, if necessary, within a range not impairing theadvantageous effects of the present invention.

By forming a coat with such an electrically conductive coating agent asdescribed above, the electrically conductive coat can be obtained.Moreover, by forming the electrically conductive coat on a substrate, anelectrically conductive film can be obtained. The electricallyconductive coating agent, the electrically conductive coat and theelectrically conductive film contain the fibrous copper microparticlesof the present invention, suppressed in the occurrence of one or moreirregularities on the surface of each thereof, and hence can prevent thevarious industrially disadvantageous problems caused by theirregularities.

Examples of the method for forming the electrically conductive coatinclude a method (liquid phase coat formation method) in which theelectrically conductive coating agent is applied to the surface of asubstrate such as a plastic film, subsequently dried, and then, ifnecessary, cured to form a coat. As the application method in the liquidphase coat formation method, for example, the following methods can beused: a roll coating method, a bar coating method, a dip coating method,a spin coating method, a casting method, a die coating method, a bladecoating method, a gravure coating method, a curtain coating method, aspray coating method and a doctor coating method.

The thickness of the electrically conductive coat may be, for example,about 0.1 to 10 μm from the viewpoint of practicability.

In order to form an electrically conductive coat or an electricallyconductive film containing the fibrous copper microparticles of thepresent invention, a method can also be used in which only the fibrouscopper microparticles of the present invention are sprayed to thesurface of a substrate such as a plastic film, and if necessary, acoating layer for protecting the sprayed fibrous copper microparticlesis formed.

EXAMPLES

Hereinafter, Examples of the present invention are described. It is tobe noted that the present invention is not limited by these Examples.

The evaluation methods and the measurement methods for the fibrouscopper microparticles and the aggregates thereof obtained in Examplesand Comparative Examples are as follows.

1. Evaluation of Reducing Compound not Reacting with Dissolved Oxygen

First, a few drops of a 10% sodium hydroxide aqueous solution were addedto 500 g of pure water to prepare an alkaline aqueous solution (watertemperature: 25° C.) the pH of which was adjusted to 10.4 (the dissolvedoxygen concentration of the alkaline aqueous solution at this time is“the dissolved oxygen concentration 1.” Specifically, the dissolvedoxygen concentration 1 is 8.3 mg/L). For the measurement of thedissolved oxygen concentration, a dissolved oxygen meter “DO-5509”(manufactured by Lutron Electronic Enterprise Co., Ltd.) was used.

Then, in an open cylindrical vessel of 7.0 cm in diameter, 100 mL of thealkaline aqueous solution was placed. Next, a reducing compound wasadded to the alkaline aqueous solution in an amount to give theconcentration of the reducing compound of 0.50 mol/L, and the reducingcompound was dissolved by stirring with a magnetic stirrer to such anextent that the aqueous solution did not swirl. While the aqueoussolution was being stirred even after the completion of dissolution, thedissolved oxygen concentration after 10 minutes from the addition of thereducing compound was measured (the dissolved oxygen concentration ofthe alkaline aqueous solution at this time is “the dissolved oxygenconcentration 2”).

On the basis of the evaluation standard for the reaction between thereducing compound and the dissolved oxygen, based on the foregoingformula (1) [(dissolved oxygen concentration retention rate)=(dissolvedoxygen concentration 2)/(dissolved oxygen concentration 1)], thereactivity between the reducing compound used in each of Examples andComparative Examples and the dissolved oxygen was evaluated. Thereducing compounds having a dissolved oxygen concentration retentionrate of 0.5 or more were evaluated as “the reducing compounds notreacting with the dissolved oxygen.”

2. Number of Fibrous Copper Microparticles Each Having One or MoreIrregularities Per 100 Fibrous Copper Microparticles

The aggregates of the fibrous copper microparticles were prepared, andwere lightly disentangled by using an ultrasonic disperser in order thatthe fibrous copper microparticles might not crowd each other. Then, theaggregates were observed with the digital microscope and an electronmicroscope (Field Emission Scanning Electron Microscope, “S-800,”manufactured by Hitachi High-Technologies Corp.), and 100 of the fibrouscopper microparticles each having a length of 1 μm or more were selectedfrom the aggregates. The surface of each of the fibrous coppermicroparticles was observed, and in a range of 1 μm in the lengthwisedirection of the fibrous body, and for each of the fibrous coppermicroparticles each having a minimum diameter portion falling in adiameter dimension range from 0.01 to 0.5 μm, the occurrence ornon-occurrence of one or more irregularities each having a dimensionaldifference of 0.02 μm or more from the minimum diameter portion wasverified. For the 100 fibrous copper microparticles, the number of thefibrous copper microparticles each undergoing the occurrence of one ormore irregularities was counted, and thus the number of the fibrouscopper microparticles each having one or more irregularities on thesurface thereof per 100 of the fibrous copper microparticles wasdetermined.

3. Average Fiber Diameter of Fibrous Copper Microparticles

In each of the 100 fibrous copper microparticles selected in theforegoing 2. the diameter of the minimum diameter portion in the totallength was taken as the fiber diameter, and each of the fiber diameterswas measured with the digital microscope and the electron microscope.The average value of the fiber diameters of the 100 fibrous coppermicroparticles was calculated, and was taken as the average fiberdiameter of the fibrous copper microparticles.

4. Average Fiber Length and Average Aspect Ratio of Fibrous CopperMicroparticles

The length of each of the 100 fibrous copper microparticles selected inthe foregoing 2. was measured with the digital microscope and theelectron microscope, and the average value of the lengths of the 100fibrous copper microparticles was calculated, and was taken as theaverage fiber length of the fibrous copper microparticles. The averageaspect ratio of the fibrous copper microparticles was derived bydividing the average fiber length of the fibrous copper microparticlesby the average fiber diameter of the fibrous copper microparticlesdetermined in the foregoing 3.

5. Full Width at Half Maximum

The aggregates of the fibrous copper microparticles obtained in each ofthe Examples and Comparative Examples were subjected to X-raydiffraction in the atmospheric air by using the wide angle X-raydiffractometer, the width of the diffraction intensity curve at half theintensity of the peak (at around 43 (deg)) corresponding to the copper(111) plane was determined by wide angle X-ray diffraction method andwas taken as the full width at half maximum.

For X-ray diffraction, the powder reflection method was adopted. Themeasurement conditions involved are as follows.

Irradiation conditions: Cu-Kα ray (voltage: 50 kV, current: 300 mA),parallel beam method (CBO unit), 25° C.

Scanning conditions: 2 deg/min, 2θ/θ continuous scanning

Goniometer radius: 285 mm

Slit width conditions: Divergence slit: 1 mm, Divergence slit length: 10mm, Scattering slit: 1 mm, Receiving slit: 0.2 mm

Filter: Nickel filter (thickness: 0.013 to 0.017 mm)

Scintillation counter: model SC-70C

Analysis software: JADE (version 7.5)

For each of the peaks, the baseline was set as shown in FIG. 5, and thepeak intensity was measured from the baseline.

6. Average Crystallite Diameter

For the aggregates of the fibrous copper microparticles obtained in eachof Examples and Comparative Examples, the average crystallite diameterwas obtained on the basis of the foregoing formula (2) [(averagecrystallite diameter) (μm)=(K×λ)/(β×cos θ)]. For β in the foregoingformula (2), the full width at half maximum obtained in the foregoing 5.was substituted.

Production of Fibrous Copper Microparticles and Aggregates ThereofExample 1

In 186 g of pure water, 108.0 g of sodium hydroxide (manufactured byNacalai Tesque, Inc.), 0.15 g of copper nitrate trihydrate (manufacturedby Nacalai Tesque, Inc.) and 0.81 g of ethylenediamine (manufactured byNacalai Tesque, Inc.) were mixed by stirring at 200 rpm at roomtemperature, and thus an aqueous solution dissolving these compounds wasprepared. The obtained aqueous solution exhibited a clear, bright bluecolor. The molar ratio between the hydroxide ion and the copper ion inthe aqueous solution was set at 4500/1.

Moreover, to the aqueous solution, 1.2 g of an ascorbic acid aqueoussolution (manufactured by Nacalai Tesque, Inc.) (4.4% by mass) (in anamount of 0.5 times the number of moles of copper ion) was added as thereducing compound, and stirred at 200 rpm for a few minutes to prepare auniform aqueous solution.

The aqueous solution prepared above was injected from the lower part ofa glass column vessel (volume: 30 mL) equipped with a jacket circulatinghot water at 70° C., in such a way that the flow rate of the aqueoussolution was controlled so as for the column passage time (regarded asthe heating time in the step (I)) to be 30 minutes. In this case, theaqueous solution was heated to 70° C. Consequently, with the heatingtime of 30 minutes, the aqueous solution turned almost colorless andtransparent. The aqueous solution having turned colorless andtransparent was successively discharged from the upper part of thecolumn vessel. The description up to here is for the step (I) of thepresent invention.

The description from here on is for the step (II) (or the step (IIa)) ofthe present invention.

In the aqueous solution after passing through the step (I), 4.8 g (anamount of 2.0 times the number of moles of copper ion) of an ascorbicacid aqueous solution (4.4% by mass) was mixed as the reducing compound.Then, successively to the step (I), the aqueous solution was injectedcontinuously from the lower part of a glass column vessel equipped witha jacket circulating hot water at 70° C. in such a way that the flowrate of the aqueous solution was controlled so as for the temperature ofthe aqueous solution to be maintained at 70° C. and so as for the columnpassage time (regarded as the temperature maintenance time in the step(II)) to be 30 minutes. As a result of allowing the aqueous solution toflow continuously, the successive precipitation of the fibrous coppermicroparticles and the aggregates thereof was visually verified.

The fibrous copper microparticles and the aggregates thereofprecipitated in the step (II) were collected by filtration underpressure of compressed nitrogen [PTFE (polytetrafluoroethylene) membranefilter having a pore size of 1 μm, manufactured by Advantec Co., Ltd.],and were once washed with an ascorbic acid aqueous solution (10% bymass), and then washed three times with pure water. Subsequently, in adryer set at 50° C., the liquid contained in the fibrous coppermicroparticles and the aggregates thereof was dried and removed, andthus the fibrous copper microparticles and the aggregates thereof ofExample 1 were obtained. The evaluation results of the fibrous coppermicroparticles and the aggregates thereof are shown in Table 1.

TABLE 1 Production steps Reducing compound Step (I) Step (II) Additionamount (number Time for turning Temperature of moles relative toTemperature Heating colorless and Temperature maintenance one mole ofcopper ion) (° C.) time (min) transparent (min) (° C.) time (min) TypeStep (I) Step (II) Example 1 70 30 30 70 30 Ascorbic acid 0.5 2.0Example 2 70 30 30 70 90 Ascorbic acid 0.5 2.0 Example 3 70 30 30 70 180Ascorbic acid 0.5 2.0 Example 4 70 30 30 70 90 Ascorbic acid 0.5 5.0Example 5 70 30 30 70 90 Ascorbic acid 0.5 100.0 Example 6 80 20 20 8090 Ascorbic acid 0.5 2.0 Example 7 60 80 80 60 260 Ascorbic acid 0.5 2.0Example 8 70 10 10 70 60 Ascorbic acid 2.5 0 Example 9 70 10 10 70 180Ascorbic acid 2.5 0 Example 10 70 30 30 70 20 Ascorbic acid 0.5 2.0Example 11 70 30 30 70 30 Erythorbic acid 0.5 2.0 Example 12 70 30 30 7030 Glucose 0.5 0.5 Comparative 70 1 1 70 10 Hydrazine 1.4 0 Example 1Comparative 70 1 1 70 <10 Hydrazine 2.0 0 Example 2 Comparative 70 1 170 5 Hydrazine 2.5 0 Example 3 Comparative 70 5 5 70 <10 Hydrazine 0.52.0 Example 4 Comparative 70 30 30 — Ascorbic acid 0.5 2.0 Example 5Shapes and properties of fibrous copper microparticles and aggregatesthereof Number of fibrous Average Average Average Average Average coppermicroparticles crystallite crystallite fiber diameter fiber lengthaspect each having one or diameter diameter/average (μm) (μm) ratio moreirregularities (μm) fiber diameter Example 1 0.07 50 714 4 0.0455 0.65Example 2 0.08 55 688 8 0.063 0.79 Example 3 0.08 55 688 6 0.076 0.95Example 4 0.09 47 522 4 0.0661 0.73 Example 5 0.10 43 430 6 0.074 0.74Example 6 0.08 53 663 4 0.077 0.96 Example 7 0.08 60 750 8 0.052 0.65Example 8 0.11 43 391 7 0.0731 0.66 Example 9 0.11 45 409 8 0.0713 0.65Example 10 0.075 48 640 5 0.0326 0.43 Example 11 0.08 52 650 5 0.05240.66 Example 12 0.10 24 240 7 0.0471 0.47 Comparative 0.13 17 131 460.0304 0.23 Example 1 Comparative 0.12 26 217 55 0.0335 0.30 Example 2Comparative 0.17 21 124 71 0.0406 0.24 Example 3 Comparative 0.45 16 3690 0.0388 0.09 Example 4 Comparative No precipitation Example 5

Examples 2 to 7

In each of Examples 2 to 7, the heating temperature and the heating time(the column passage time) in the step (I), the temperature maintenancetime (the column passage time) in the step (II) and the addition amountof the reducing compound in the step (II) were altered as described inTable 1. In each of Examples 2 to 7, fibrous copper microparticles andthe aggregates thereof were obtained otherwise in the same manner as inExample 1. The evaluation results of these fibrous copper microparticlesand the aggregates thereof are shown in Table 1.

Examples 8 and 9

In each of Examples 8 and 9, the heating time (the column passage time)in the step (I) and the temperature maintenance time (the column passagetime) in the step (II) were altered as described in Table 1, the amountof the reducing compound beforehand added in the step (I) was altered asdescribed in Table 1, and no reducing compound was added in the step(II). In each of Examples 8 and 9, fibrous copper microparticles and theaggregates thereof were obtained otherwise in the same manner as inExample 1. The evaluation results of these fibrous copper microparticlesand the aggregates thereof are shown in Table 1.

Example 10

The temperature maintenance time (the column passage time) in the step(II) was set at 20 minutes. Fibrous copper microparticles and theaggregates thereof were obtained otherwise in the same manner as inExample 1. The evaluation results of the fibrous copper microparticlesand the aggregates thereof are shown in Table 1.

Example 11

The reducing compound added in the step (I) and the step (II) waserythorbic acid. Fibrous copper microparticles and the aggregatesthereof were obtained otherwise in the same manner as in Example 1. Theevaluation results of the fibrous copper microparticles and theaggregates thereof are shown in Table 1.

Example 12

The reducing compound added in the step (I) and the step (II) wasglucose, and the amount of the glucose added in the step (II) was 0.5times the number of moles of copper ion. Fibrous copper microparticlesand the aggregates thereof were obtained otherwise in the same manner asin Example 1. The evaluation results of the fibrous coppermicroparticles and the aggregates thereof are shown in Table 1.

Comparative Examples 1 to 3

In each of Comparative Examples 1 to 3, the heating time (the columnpassage time) in the step (I) and the temperature maintenance time (thecolumn passage time) in the step (II) were altered as described in Table1, hydrazine (added as hydrazine monohydrate, manufactured by Wako PureChemical Industries, Ltd.) was added in place of ascorbic acid as thereducing compound in an amount described in Table 1 in the step (I), andno reducing compound was added in the step (II). Fibrous coppermicroparticles and the aggregates thereof were obtained otherwise in thesame manner as in Example 1. In the step (II), the prepared aqueoussolution was injected from the lower part of a glass column vessel(volume: 30 mL) equipped with a jacket circulating hot water at 70° C.in such a way that the flow rate of the aqueous solution was controlledso as for the column passage time (regarded as the heating time) to be20 minutes. The evaluation results of these fibrous coppermicroparticles and the aggregates thereof are shown in Table 1.

Comparative Example 4

The heating time (the column passage time) in the step (I) and thetemperature maintenance time (the column passage time) in the step (II)were altered as described in Table 1, and hydrazine (added as hydrazinemonohydrate, manufactured by Wako Pure Chemical Industries, Ltd.) wasadded in place of ascorbic acid as the reducing compound in the amountsdescribed in Table 1 in the step (I) and in the step (II). Fibrouscopper microparticles and the aggregates thereof were obtained otherwisein the same manner as in Example 1. The evaluation results of thesefibrous copper microparticles and the aggregates thereof are shown inTable 1.

Comparative Example 5

The same operations as in Example 1 were performed up to the step (I),and then, 4.8 g (an amount of 2.0 times the number of moles of copperion) of an ascorbic acid aqueous solution (4.4% by mass), a reducingcompound, was mixed with the aqueous solution. Then, the mixed aqueoussolution was rapidly cooled to 30° C. in 10 minutes by allowing theaqueous solution to pass through a glass column vessel equipped with ajacket circulating cool water (20° C.). It is to be noted that in Table1, “-” means rapid cooling with cool water and the omission of the step(II).

In each of Examples 1 to 12, the aqueous solution was heated in the step(I), the temperature maintenance time in the step (II) was set at 20minutes or more, and the fibrous copper microparticles were continuouslyproduced by using a column vessel equipped with a hot water jacket.Consequently, as shown in Table 1, the number of the fibrous coppermicroparticles each having one or more irregularities on the surfacethereof was 10 or less per 100 of the fibrous copper microparticles,showing that the occurrence of one or more irregularities on the surfaceof each of the fibrous copper microparticles was sufficientlysuppressed.

In Examples 1 to 9, 11 and 12, the temperature maintenance time in thestep (II) was set at “30 minutes or more,” and the fibrous coppermicroparticles were produced continuously by using the column vesselequipped with a hot water jacket. Consequently, as shown in Table 1, thefibrous copper microparticles having a fiber diameter of 0.01 to 0.5 μmand an average aspect ratio of 10 or more were obtained. Moreover, theaggregates of the fibrous copper microparticles having an averagecrystallite diameter of 0.045 to 0.1 μm and an average fiber diameter of0.05 to 0.15 μm were obtained. Moreover, the aggregates of the fibrouscopper microparticles having an average fiber diameter of 0.05 to 0.15μm and an average crystallite diameter of 0.45 or more times the averagefiber diameter were obtained. In other words, in the step (II), thetemperature of the aqueous solution passing through the step (I) wasmaintained for “30 minutes or more,” and consequently the aggregates ofthe fibrous copper microparticles having a large average crystallitediameter (0.045 to 0.1 μm) were obtained.

In particular, in Examples 1 to 7, 11 and 12 (Examples in which thereducing compound was further added in the step (II)), as compared withExamples 8 and 9 (Examples in which the reducing compound was not addedin the step (II), but the total amount of the reducing compound wasadded in the step (I)), the aggregates of the fibrous coppermicroparticles, having a small distribution of largish fiber diameter,and having a somewhat smaller average fiber diameter (specifically, 0.10μm or less) were obtained.

In each of Comparative Examples 1 to 4, the color of the solutionchanged from blue in the step (I) in a short time, to yield a colorlessand transparent solution; then, within 10 minutes, the fibrous coppermicroparticles and the aggregates thereof were completely precipitated.Accordingly, it was impossible to continuously precipitate the fibrouscopper microparticles and the aggregates thereof by setting thetemperature maintenance time at 20 minutes or more. In each ofComparative Examples 1 to 4, the number of the fibrous coppermicroparticles each having one or more irregularities on the surfacethereof exceeded 10 per 100 of the fibrous copper microparticles, andthus it was impossible to suppress the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles.

In each of Comparative Examples 1 to 4, in the step (II), thetemperature of the aqueous solution passing through the step (I) was notable to be maintained for “30 minutes or more,” and hence, only theaggregates of the fibrous copper microparticles having an averagecrystallite diameter of 0.0304 to 0.0406 μm falling outside the rangespecified in the present invention (0.045 to 0.1 μm) were obtained. Onlythe aggregates of the fibrous copper microparticles having an averagecrystallite diameter of less than 0.45 times the average fiber diameterwere obtained.

Comparative Example 5 adopted a production process not including thestep (II) and performing a rapid cooling, and hence was unable to obtaineven the aggregates of the fibrous copper microparticles.

Example 13

In 186 g of pure water, 108.0 g of sodium hydroxide (manufactured byNacalai Tesque, Inc.), 0.15 g of copper nitrate trihydrate (manufacturedby Nacalai Tesque, Inc.) and 0.81 g of ethylenediamine (manufactured byNacalai Tesque, Inc.) were mixed by stirring at 200 rpm at roomtemperature, and thus an aqueous solution dissolving these compounds wasprepared. The obtained aqueous solution exhibited a clear, bright bluecolor. The molar ratio between the hydroxide ion and the copper ion inthe aqueous solution was set at 4500/1.

Moreover, to the aqueous solution, 1.2 g of an ascorbic acid aqueoussolution (manufactured by Nacalai Tesque, Inc.) (4.4% by mass) (in anamount of 0.5 times the number of moles of copper ion) was added as thereducing compound, and stirred at 200 rpm for a few minutes to prepare auniform aqueous solution.

The aqueous solution prepared above was injected from the lower part ofa glass column vessel (volume: 30 mL) equipped with a jacket circulatinghot water at 80° C., in such a way that the flow rate of the aqueoussolution was controlled so as for the column passage time (regarded asthe heating time in the step (I)) to be 20 minutes. In this case, theaqueous solution was heated to 80° C. Consequently, with the heatingtime of 20 minutes, the aqueous solution turned almost colorless andtransparent. The aqueous solution having turned colorless andtransparent was successively discharged from the upper part of thecolumn vessel. The description up to here is for the step (I) (or thestep (Ia)) of the present invention.

The description from here on is for the step (III) (or the step (IIIa))of the present invention.

In the aqueous solution after passing through the step (I), 4.8 g (anamount of 2.0 times the number of moles of copper ion) of an ascorbicacid aqueous solution (4.4% by mass) was mixed as the reducing compound.Then, successively to the step (I), the aqueous solution was injectedfrom the lower part of a glass column vessel not equipped with a heatingjacket and allowed to flow continuously through the column vessel, andthus the temperature of the aqueous solution was decreased to 70° C.after 10 minutes and to 60° C. after 30 minutes. In other words, in thestep (III), the time required for decreasing the temperature of theaqueous solution by 20° C. was 30 minutes. In the mentioned slow coolingprocess, the successive precipitation of the fibrous coppermicroparticles and the aggregates thereof was visually verified.

The fibrous copper microparticles and the aggregates thereofprecipitated in the step (III) were collected by filtration underpressure of compressed nitrogen [PTFE membrane filter having a pore sizeof 1 μm, manufactured by Advantec Co., Ltd.], and were once washed withan ascorbic acid aqueous solution (10% by mass), and then washed threetimes with pure water. Subsequently, in a dryer set at 50° C., theliquid contained in the fibrous copper microparticles and the aggregatesthereof was dried and removed, and thus the fibrous coppermicroparticles and the aggregates thereof of Example 13 were obtained.The evaluation results of the fibrous copper microparticles and theaggregates thereof are shown in Table 2.

TABLE 2 Production steps Reducing compound Step (I) Step (III) Additionamount (number Time for turning Time required for decreasing of molesrelative to Temperature Heating colorless and by 20° C. temperature ofone mole of copper ion) (° C.) time (min) transparent (min) aqueoussolution (min) Type Step (I) Step (III) Example 13 80 20 20 30 Ascorbicacid 0.5 2.0 Example 14 80 20 20 30 Ascorbic acid 0.5 1.0 Example 15 8045 45 30 Ascorbic acid 0.25 0.5 Example 16 70 30 30 45 Ascorbic acid 0.52.0 Example 17 70 30 30 30 Glucose 0.5 0.5 Comparative 70 30 1 —Hydrazine 2.5 0 Example 6 Shapes and properties of fibrous coppermicroparticles and aggregates thereof Number of fibrous Average AverageAverage Average Average copper microparticles crystallite crystallitefiber diameter fiber length aspect each having one or diameterdiameter/average (μm) (μm) ratio more irregularities (μm) fiber diameterExample 13 0.06 58 967 4 0.0166 0.28 Example 14 0.07 65 929 3 0.02930.42 Example 15 0.07 55 786 4 0.0194 0.28 Example 16 0.06 48 800 40.0275 0.46 Example 17 0.09 25 277 5 0.0180 0.20 Comparative 0.20 18 9053 0.0420 0.21 Example 6

Examples 14 to 17

In each of Examples 14 to 17, the heating temperature and the heatingtime (the column passage time) in the step (I), the time required in thestep (III), for decreasing by 20° C. the temperature of the aqueoussolution after passing through the step (I), and the addition amountsand the type (as the reducing compound in Example 17, glucose(manufactured by Nacalai Tesque, Inc.) was used) of the reducingcompound in the step (I) and the step (III) were altered as described inTable 2. Fibrous copper microparticles and the aggregates thereof wereobtained otherwise in the same manner as in Example 13. The evaluationresults of these fibrous copper microparticles and the aggregatesthereof are shown in Table 2.

Comparative Example 6

The heating temperature and the heating time (the column passage time)in the step (I), the time required in the step (III), for decreasing by20° C. the temperature of the aqueous solution after passing through thestep (I), and the addition amounts and the type (hydrazine (added ashydrazine monohydrate, manufactured by Wako Pure Chemical Industries,Ltd.)) of the reducing compound in the step (I) and the step (III) werealtered as described in Table 2. Fibrous copper microparticles and theaggregates thereof were obtained otherwise in the same manner as inExample 13. The evaluation results of the fibrous copper microparticlesand the aggregates thereof are shown in Table 2.

It is to be noted that in Table 2, “-” means the start of theprecipitation of the fibrous copper microparticles and the aggregatesthereof, before the aqueous solution was subjected to the slow cooling.

In each of Examples 13 to 17, due to the fact that “in the step (I), theaqueous solution was heated” and the fact that “in the step (III), thetemperature of the aqueous solution after passing through the step (I)was decreased by 20° C. by cooling over a time of “15 minutes or more”immediately after the start of the cooling”, as shown in Table 2, thenumber of the fibrous copper microparticles each having one or moreirregularities on the surface thereof was 10 or less per 100 of thefibrous copper microparticles, and thus the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles was sufficiently suppressed.

Also, in each of Examples 13 to 17, due to the fact that in the step(III), the temperature of the aqueous solution after passing through thestep (I) was decreased by 20° C. by cooling over a time of “15 minutesor more” immediately after the start of the cooling, as shown in Table2, the aggregates of the fibrous copper microparticles having an averagecrystallite diameter of 0.015 to 0.03 μm, an average fiber diameter of0.03 to 0.1 μm and an average aspect ratio of 10 or more was able to beobtained.

In Comparative Example 6, immediately after the step (I), the fibrouscopper microparticles and the aggregates thereof precipitated, andcompletely precipitated before the aqueous solution was subjected to theslow cooling in the step (III). Accordingly, in Comparative Example 6,the number of the fibrous copper microparticles each having one or moreirregularities on the surface thereof exceeded 10 (was 53) per 100 ofthe fibrous copper microparticles, and the occurrence of one or moreirregularities on the surface of each of the fibrous coppermicroparticles fell outside the range (10 or less) specified in thepresent invention. Moreover, in the aggregates of the fibrous coppermicroparticles obtained in Comparative Example 6, the averagecrystallite diameter was 0.042 μm and the average fiber diameter was 0.2μm, both deviating the ranges specified in the present invention (theaverage crystallite diameter range from 0.015 to 0.03 μm and the averagefiber diameter range from 0.03 to 0.1 μm).

FIG. 6 shows an observation view (magnification factor: 40000) obtainedby observing with a SEM the fibrous copper microparticles obtained inExample 13. As can be seen from FIG. 6, the fibrous coppermicroparticles obtained in Example 13 were sufficiently suppressed inthe occurrence of one or more irregularities on the surface of eachthereof.

FIG. 7 shows an observation view (magnification factor: 40000) obtainedby observing with a SEM the fibrous copper microparticles obtained inComparative Example 2. As can be seen from FIG. 7, in the fibrous coppermicroparticles obtained in Comparative Example 2, a large number of thefibrous copper microparticles having one or more irregularities on thesurface of each thereof were formed.

REFERENCE SIGNS LIST

-   1 Fibrous copper microparticle-   2 Range of 1 μm in lengthwise direction of fibrous body-   3 Irregularity defined in the present invention-   4 Maximum diameter portion of fibrous copper microparticle-   5 Minimum diameter portion of fibrous copper microparticle-   6 Dimensional difference between maximum diameter portion and    minimum diameter portion

1. Fibrous copper microparticles, wherein a number of fibrous coppermicroparticles each including one or more irregularities each having adimensional difference of 0.02 μm or more, in a range of 1 μm in alengthwise direction of a fibrous body, between a maximum diameterportion of the fibrous body and a minimum diameter portion of thefibrous body falling in a diameter dimension range of 0.01 to 0.5 μm,and each having a length of 1 μm or more is 10 or less per 100 of thefibrous copper microparticles.
 2. The fibrous copper microparticlesaccording to claim 1, wherein for each of the fibrous coppermicroparticles, a fiber diameter is 0.01 to 0.5 μm, and an aspect ratiois 10 or more.
 3. Aggregates of fibrous copper microparticles, formed byallowing the fibrous copper microparticles according to claim 1 toaggregate, wherein an average crystallite diameter is 0.045 to 0.1 μmand an average fiber diameter is 0.05 to 0.15 μm.
 4. Aggregates offibrous copper microparticles, formed by allowing the fibrous coppermicroparticles according to claim 1 to aggregate, wherein the averagefiber diameter is 0.05 to 0.15 μm, and an average crystallite diameteris 0.45 or more times the average fiber diameter.
 5. Aggregates offibrous copper microparticles, formed by allowing the fibrous coppermicroparticles according to claim 1 to aggregate, wherein the averagecrystallite diameter is 0.015 to 0.03 μm, and the average fiber diameteris 0.03 to 0.1 μm.
 6. A process for producing fibrous coppermicroparticles, wherein the production process is a process forproducing the fibrous copper microparticles according to claim 1; andthe production process comprises a following step (I) and a followingstep (II) or (III), in this order: the step (I) of heating, to 50 to100° C., an aqueous solution containing copper ion, an alkalinecompound, a nitrogen-containing compound capable of forming a stablecomplex with copper ion and a reducing compound; the step (II) ofmaintaining for 20 minutes or more a temperature of the aqueous solutionafter passing through the step (I), and continuously precipitating thefibrous copper microparticles; the step (III) of cooling the temperatureof the aqueous solution after passing through the step (I) to decreasethe temperature thereof by 20° C. over a period of time of 15 minutes ormore from immediately after a start of cooling, and continuouslyprecipitating the fibrous copper microparticles.
 7. The process forproducing fibrous copper microparticles according to claim 6, wherein inthe step (II) or (III), the reducing compound is further added to theaqueous solution.
 8. A process for producing aggregates of fibrouscopper microparticles, wherein the production process is a process forproducing the aggregates of the fibrous copper microparticles accordingto claim 3; and the production process comprises the following step (I)and a following step (IIa), in this order, and continuously precipitatesthe fibrous copper microparticles or the aggregates of the fibrouscopper microparticles: the step (I) of heating to 50 to 100° C. theaqueous solution containing copper ion, an alkaline compound, anitrogen-containing compound capable of forming a stable complex withcopper ion and a reducing compound; the step (IIa) of maintaining for 30minutes or more the temperature of the aqueous solution after passingthrough the step (I).
 9. A process for producing aggregates of fibrouscopper microparticles, wherein the production process is a process forproducing the aggregates of the fibrous copper microparticles accordingto claim 5; and the production process comprises a following step (Ia)and a following step (IIIa), in this order, and continuouslyprecipitates the fibrous copper microparticles or the aggregates of thefibrous copper microparticles: the step (Ia) of heating to 65 to 100° C.the aqueous solution containing copper ion, an alkaline compound, anitrogen-containing compound capable of forming a stable complex withcopper ion and a reducing compound; the step (IIIa) of decreasing thetemperature of the aqueous solution after passing through the step (Ia)by 20° C. over a period of time of 15 minutes or more from immediatelyafter a start of cooling.
 10. The process for producing aggregates offibrous copper microparticles according to claim 8, wherein in the step(IIa), the reducing compound is further added to the aqueous solution.11. The process for producing aggregates of fibrous coppermicroparticles according to claim 9, wherein in the step (IIIa), thereducing compound is further added to the aqueous solution.
 12. Theprocess for producing fibrous copper microparticles according to claim6, wherein as the reducing compound, one or more selected from ascorbicacid, erythorbic acid and glucose are used.
 13. The process forproducing aggregates of fibrous copper microparticles according to claim8, wherein as the reducing compound, one or more selected from ascorbicacid, erythorbic acid and glucose are used.